Method and system for optimization of an optical transmission signal which is modulated with a binary data signal

ABSTRACT

A method and system for optimization of an optical transmissions signal which is modulated with a binary data signal, wherein a control device ensures that the operating point and the modulation signal of a Mach-Zehnder modulator are set optimally, with the fundamental frequency and/or the first harmonic frequency of the transmission signal being selected for this purpose, and an optimum setting is reached when the amplitude of the fundamental frequency has reached a maximum value and the amplitude of the harmonic frequency has reached a minimum value.

BACKGROUND OF THE INVENTION

Mach-Zehnder modulators (MZI Mach-Zehnder interferometers) are used inoptical transmission systems whose data rates are 10 Gbit/s or moresince, at these high data rates and with the present-day state of theart, neither direct modulation of a laser nor modulation usingelectroabsorption modulators is expedient. Generally, in addition to themodulation signal, MZI modulators require a bias voltage for setting theoperating point in order to achieve a balanced output signal and, hence,a balanced eye shape for the received signal. Any deviation of theoperating point from this value leads to distortion in the opticaltransmission signal and, hence, to greater error rates and/or to areduced range. Like all interferometer arrangements, which reactextremely sensitively to a very small optical path length change, theoperating point also varies with the environmental conditions, in mostavailable modulators.

An MZI modulator is described, for example, in the “Designer's Guide toExternal Modulation”, UTP, 1289 Blue Hills Avenue, Bloomfield, Conn.,pages 4-6. When the modulator is being fully driven, any change in theoperating point leads to overdriving, as a result of which, afterinitially rising, the optical power falls once again, despite thecontrol signal increasing, during the transition from blocking afterswitching on from “0” to “1”. In the event of overdriving, a transmitted“1 bit” peaks in the region of the rising 1 flank, resulting in arelatively high frequency structure in which some of the spectral poweris contained in higher frequencies in particular; for example, at twicethe fundamental frequency of the data signal, or at the data rate.

Patent specification U.S. Pat. No. 5,710,653 discloses a transmissionsystem having a module for external modulation of a signal, in whichharmonic frequencies of the modulated signal are suppressed. A firstmethod uses two Mach-Zender interferometers, arranged in parallel, asmodulators, to whose inputs the signal to be modulated is supplied withdifferent amplitudes (80% and 20%), and whose two modulated outputsignals are combined such that a harmonic frequency from the secondoutput signal is isolated from the fundamental frequency, is invertedand then amplified such that the harmonic frequencies in the resultantmodulated signal compensate for one another by addition between thefirst modulated output signal and the processed isolated harmonicfrequency of the second modulated output signal. In the second method,only one modulator is used in order to suppress second-order andthird-order harmonic frequencies in the modulated signal. In this case,a distortion network is required to suppress the third-order harmonicfrequency in addition to controlling the operating point of themodulator. These two methods suppress third-order harmonic frequencies.An additional control loop is provided for setting the operating pointof the modulator or modulators in phase quadrature in order to eliminatethe second-order harmonic frequency.

Patent Specification U.S. Pat. No. 5,629,792 discloses a furtherarrangement and method for modulation of a signal, with closed-loopcontrol of the operating point of the modulator. The closed-loop controlcontrols the operating point of the modulator by measuring the powerlevel of the fundamental frequency of the modulated signal emitted bythe modulator. This closed-loop process does not overcome, nor does itminimize, the influences of interference harmonic frequencies in themodulated signal.

An object of the present invention is, therefore, to specify a methodwhich provides as simple a solution approach as possible for suppressionof harmonic frequencies. Another aim is to specify a suitable system fordoing the same.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the Figures.

SUMMARY OF THE INVENTION

This object is achieved by using the signal at the harmonic frequencywhich, in each case, has been filtered out to derive a control signalwhich controls the operating point of the modulator, such that the atleast one harmonic frequency reaches a minimum amplitude or power.

The advantage of this solution is that it requires considerably lesscomplexity than the prior art for suppressing harmonic frequencies.

The method makes use of an effect, which occurs when the modulator isoverdriven, for regulating the operating point. It is particularlyadvantageous in this case to combine operating point control withcontrol of the modulation signal. All major parameters are kept constantby the control process.

It is also advantageous for the control criteria to be obtained at thereceiving end, and to be transmitted via a service channel. This alsomakes it possible to partially compensate for distortion caused by thetransmission path.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the Figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an outline circuit diagram of a system according to thepresent invention.

FIG. 2 shows a waveform of the associated control voltage.

FIG. 3 shows a variant of the system according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The system illustrated in FIG. 1 contains a Mach-Zehnder modulator (MZI)4, to which an optical signal OS is supplied from a laser 3. Themodulator 4 is modulated with a data signal DS, which is supplied from adata source, as a modulation signal U_(DAT), via a controllableamplifier 2. The modulated optical transmission signal OSM istransmitted. A small portion of the signal is supplied via an opticalsplitter 5 to an optoelectrical transducer 6 with a downstream amplifier7, and is demodulated. The electrical data signal DS1 recovered in thisway essentially contains the modulation signal U_(DAT) or data signalDS. The data signal DS1 is supplied via an amplifier 7 to two filters,the bandpass filters 8 and 10. The first bandpass filter 8 filters thefundamental frequency GW out of the data signal DS1; that is to say, itspass frequency is at half the bit rate. A low-pass-filtered 01 bitsequence essentially results in a sinusoidal voltage at a frequencycorresponding to half the data rate (a derived data signal also may beused instead of the NRZ data signal). The output voltage from the firstbandpass filter 8 is supplied directly or via a measurement transducer9, such as a rectifier or a power measurement device, as a controlsignal U_(R1) to a control device 12. In the exemplary embodiment, asecond bandpass filter 10 is provided and tuned to a harmonic frequencyOW, preferably the first. Its output voltage is also supplied directlyor via a second measurement transducer 11 as a further control signalU_(R2) to the control device 12. Additional bandpass filters also may beprovided for filtering out further harmonic frequencies, and theiroutput voltages can be combined. The control device produces, via aregulator 17, a control signal U_(BIAS), which governs the operatingpoint of the modulator 4.

As already mentioned, the first bandpass filter BP1 filters out thefundamental frequency GW. Deviations from the operating point oroverdriving caused by an excessively large modulation signal lead to areduction in the amplitude of the fundamental frequency (sinusoidalsignal), since the harmonics which then occur result in the fundamentalfrequency spectral component decreasing. A corresponding situationapplies to the control signal U_(R1) obtained from the sinusoidalsignal. An opposite situation applies to the wave form for the harmonicfrequencies. Their amplitudes and, thus, the amplitudes of the controlsignals U_(R2), . . . increase when overdriving occurs.

The diagram illustrated in FIG. 2 shows the relationship between thepower levels P of the fundamental frequency GW and of the first harmonicfrequency OW, and the operating point of the modulator. The power(amplitude) of the fundamental frequency has a clear optimum in theregion of the ideal operating point. Here, the amplitude of the harmonicfrequency (of the harmonic frequencies) is at a minimum which is morestrongly dependent on the operating point and which is particularlyhighly suitable for optimization of the operating point. It isparticularly advantageous to combine the two control signals U_(R1) andU_(R2) for operating point control, such as by addition, with one ofthese signals being inverted, since this leads to the controlcharacteristic having a steeper profile. FIG. 2 shows the main valuesfor the operating point control signal (bias voltage). The optimumoccurs at about 4.8 V.

A control device can at the same time be used to convert the data signalDS emitted from the data source to an optimum modulation signal U_(DAT)by using a further control signal U_(MOD) to control the amplifier 2,this optimum modulation signal U_(DAT) being that which leads to atransmission signal with the maximum amplitude and the minimum harmoniccontent (maximum modulation level). It is sufficient to use thefundamental frequency for control purposes in order to maximize themodulation signal U_(DAT).

During a control process, the control device can be used to producechanges in the control signals U_(BIAS) and U_(MOD) in both directionson a trial basis in order to reach the respective optimum setting. Theoperating point U_(BIAS) (control signal/bias voltage) and themodulation signal U_(DAT) may, for example, be adjusted alternately. Thefrequency of the operating point and/or the amplitude of the modulationsignal likewise may be swept (periodic variation by a small amount viafrequency-sweep voltages U_(W1), U_(W2)) in order to determine themathematical sign of any control error, with the control process beingcarried out based on the lock-in principle.

It is, of course, also possible to control the amplitude of the lasersignal OS and, hence, the amplitude of the transmission signal OSM.

FIG. 3 shows a variant in which the control signals U_(R1), U_(R2) arederived from the received signal OEM at the end of a transmission path(at the receiving end). The control process also can, in this way, takeaccount of the line characteristics. The demodulation of the receivedsignal OEM is carried out in a receiving device 20. The demodulated datasignal DS1 is once again evaluated via filters 8, 10 and is converted bythe measurement transducers 9, 11 to control signals U_(R1), U_(R2)which, after inversion of one control signal via an inverting amplifier13, are combined in an adder 16 to form a resultant control signalU_(R).

The control device 12 may be arranged at the receiving end or at thetransmitting end. In this exemplary embodiment, the combined controlsignal U_(R) is transmitted via a service channel 22 to the controldevice 12 arranged at the transmitting end, in order to optimize theoperating point and/or the amplitude of the modulation signal.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the present invention as set forth in the hereafter appended claims.

What is claimed is:
 1. A method for amplitude modulation of an opticalsignal with a binary data signal, the method comprising the steps of:supplying both the optical signal and the binary data signal to amodulator, whose operating point is adjustable, for producing an opticaltransmission signal; demodulating the transmission signal or ameasurement signal, which is tapped off the transmission signal;converting the demodulated signal to a recovered binary data signal;selecting by filtering out from the recovered data signal both afundamental frequency and at least one harmonic frequency; deriving fromthe recovered binary data signal a fundamental frequency control signaland the at least one harmonic frequency control signal; and controllingthe operating point of the modulator for optimum adjustment such thatthe fundamental frequency reaches for a maximum amplitude or power andthe at least one of the harmonic frequencies reaches for a minimumamplitude or power.
 2. A method for amplitude modulation of an opticalsignal with a binary data signal as claimed in claim 1, the methodfurther comprising the steps of: converting the binary data signal to amodulation signal which is supplied to the modulator; and regulating anamplitude of the modulation signal at a value at which one of thefundamental frequency and a control signal formed from the fundamentalfrequency and the inverted harmonic frequency reaches a maximum.
 3. Amethod for amplitude modulation of an optical signal with a binary datasignal as claimed in claim 2, the method further comprising the step ofvarying one of the operating point and an amplitude of the modulationsignal on a trial basis until the fundamental frequency has reached amaximum value.
 4. A method for amplitude modulation of an optical signalwith a binary data signal as claimed in claim 3, wherein the operatingpoint and the modulation signal are optimally set alternately.
 5. Amethod for amplitude modulation of an optical signal with a binary datasignal as claimed in claim 2, the method further comprising the step ofvarying one of the operating point and an amplitude of the modulationsignal on a trial basis until the fundamental frequency has reached amaximum value and the harmonic frequency has reached a minimum value atthe same time.
 6. A method for amplitude modulation of an optical signalwith a binary data signal as claimed in claim 5, wherein the operatingpoint and the modulation signal are optimally set alternately.
 7. Amethod for amplitude modulation of an optical signal with a binary datasignal as claimed in claim 2, wherein the operating point control signaland the amplitude of the modulation signal are optimized by sweepingrespective frequencies of the operating point control signal and themodulation signal, and by control being carried out in each case basedon a lock-in principle.
 8. A method for amplitude modulation of anoptical signal with a binary data signal as claimed in claim 1, themethod further comprising the steps of: producing the recovered datasignal by demodulation of an optical received signal; deriving controlsignals from the recovered data signal; supplying the control signals toa control device arranged at a transmission end; and transmittingcontrol signals via the control device and via a service channel to themodulator.
 9. A method for amplitude modulation of an optical signalwith a binary data signal as claimed in claim 1, the method furthercomprising the steps of: producing the recovered data signal bydemodulation of an optical received signal; deriving controlled signalsfrom the recovered data signal; supplying the control signals to acontrol device arranged at a transmission end; and transmitting controlsignals to a control device arranged at a receiving end, the controldevice controlling the modulator via a service channel.
 10. A system foramplitude modulation of an optical signal with a binary data signal,comprising: a modulator with an adjustable operating point, themodulator having an output at which a modulated transmission signal isemitted based on the optical signal and the binary data signal; ademodulator, in a control loop, through which one of the transmissionsignal or a measurement signal which corresponds to the transmissionsignal is supplied in order to obtain a recovered binary data signalfrom it; a first filter, in the control loop, which selects afundamental frequency of the recovered binary data signal; at least onesecond filter, in the control loop, which selects at least one harmonicfrequency of the recovered binary data signal; at least one measurementtransducer, in the control loop, for obtaining control signals from atleast the first and second filters; and a control device, in the controlloop, for controlling the operating point of the modulator such that theamplitude of the fundamental frequency reaches for a maximum value; andthe amplitude of the at least one harmonic frequency reaches for aminimum value.
 11. A system for amplitude modulation of an opticalsignal with a binary data signal as claimed in claim 10, furthercomprising an adjustable amplifier for converting the binary data to amodulation signal, wherein the modulation signal is regulated at amaximum value at which the amplitude of the fundamental frequency is ata maximum.
 12. A system for amplitude modulation of an optical signalwith a binary data signal as claimed in claim 10, further comprising anadjustable amplifier for converting the data signal to a modulationsignal, wherein the modulation signal is regulated at a maximum value atwhich the amplitude of the fundamental frequency is at a maximum and theamplitude of the at least one harmonic frequency is at a minimum.
 13. Amethod for amplitude modulation of an optical signal with a binary datasignal, the method comprising the steps of: supplying both the opticalsignal and the binary data signal to a modulator for producing atransmission signal whose operating point is adjustable; demodulatingone of the transmission signal and a measurement signal, which is tappedoff the transmission signal; converting the demodulated signal to arecovered data signal; filtering out at least one harmonic frequency ofthe recovered data signal; converting the data signal to a modulationsignal which is supplied to the modulator; regulating an amplitude ofthe modulation signal at a value at which one of the fundamentalfrequency and a control signal formed from the fundamental frequency andthe inverted harmonic frequency reaches a maximum; using a signal at theat least one harmonic frequency, which has been filtered out, to derivea control signal which controls an operating point of the modulator suchthat the at least one harmonic frequency reaches one of a minimumamplitude and a minimum power; and varying one of the operating pointand an amplitude of the modulation signal on a trial basis until thefundamental frequency has reached a maximum value.
 14. A method foramplitude modulation of an optical signal with a binary data signal asclaimed in claim 13, the method further comprising the steps of:selecting both a fundamental frequency and the at least one harmonicfrequency from the recovered data signal; and using control signalsderived from the fundamental frequency and the at least one harmonicfrequency for optimum adjustment of the operating point.
 15. A methodfor amplitude modulation of an optical signal with a binary data signalas claimed in claim 13, the method further comprising the step ofvarying one of the operating point and an amplitude of the modulationsignal on a trial basis until the fundamental frequency has reached amaximum value and the harmonic frequency has reached a minimum value atthe same time.
 16. A method for amplitude modulation of an opticalsignal with a binary data signal as claimed in claim 15, wherein theoperating point and the modulation signal are optimally set alternately.17. A method for amplitude modulation of an optical signal with a binarydata signal as claimed in claim 13, wherein the operating point and themodulation signal are optimally set alternately.
 18. A method foramplitude modulation of an optical signal with a binary data signal asclaimed in claim 13, wherein the operating point control signal and theamplitude of the modulation signal are optimized by sweeping respectivefrequencies of the operating point control signal and the modulationsignal, and by control being carried out in each case based on a lock-inprinciple.
 19. A method for amplitude modulation of an optical signalwith a binary data signal as claimed in claim 13, the method furthercomprising the steps of: producing the recovered data signal bydemodulation of an optical received signal; deriving control signalsfrom the recovered data signal; supplying the control signals to acontrol device arranged at a transmission end; and transmitting controlsignals via the control device and via a service channel to themodulator.
 20. A method for amplitude modulation of an optical signalwith a binary data signal as claimed in claim 13, the method furthercomprising the steps of: producing the recovered data signal bydemodulation of an optical received signal; deriving controlled signalsfrom the recovered data signal; supplying the control signals to acontrol device arranged at a transmission end; and transmitting controlsignals to a control device arranged at a receiving end, the controldevice controlling the modulator via a service channel.
 21. A method foramplitude modulation of an optical signal with a binary data signal, themethod comprising the steps of: supplying both the optical signal andthe binary data signal to a modulator for producing a transmissionsignal whose operating point is adjustable; demodulating thetransmission signal; converting the demodulated signal to a recovereddata signal; deriving control signals from the recovered data signal;supplying the control signals to a control device arranged at atransmission end; transmitting control signals via the control deviceand via a service channel to the modulator; filtering out at least oneharmonic frequency of the recovered data signal; and using a signal atthe at least one harmonic frequency, which has been filtered out, toderive a control signal which controls an operating point of themodulator such that the at least one harmonic frequency reaches one of aminimum amplitude and a minimum power.
 22. A method for amplitudemodulation of an optical signal with a binary data signal as claimed inclaim 21, the method further comprising the steps of: selecting both afundamental frequency and the at least one harmonic frequency from therecovered data signal; and using control signals derived from thefundamental frequency and the at least one harmonic frequency foroptimum adjustment of the operating point.
 23. A method for amplitudemodulation of an optical signal with a binary data signal as claimed inclaim 21, the method further comprising the steps of: converting thebinary data signal to a modulation signal which is supplied to themodulator; and regulating an amplitude of the modulation signal at avalue at which one of the fundamental frequency and a control signalformed from the fundamental frequency and the inverted harmonicfrequency reaches a maximum.
 24. A method for amplitude modulation of anoptical signal with a binary data signal as claimed in claim 23, themethod further comprising the step of varying one of the operating pointand an amplitude of the modulation signal on a trial basis until thefundamental frequency has reached a maximum value.
 25. A method foramplitude modulation of an optical signal with a binary data signal asclaimed in claim 24, wherein the operating point and the modulationsignal are optimally set alternately.
 26. A method for amplitudemodulation of an optical signal with a binary data signal as claimed inclaim 23, the method further comprising the step of varying one of theoperating point and an amplitude of the modulation signal on a trialbasis until the fundamental frequency has reached a maximum value andthe harmonic frequency has reached a minimum value at the same time. 27.A method for amplitude modulation of an optical signal with a binarydata signal as claimed in claim 26, wherein the operating point and themodulation signal are optimally set alternately.
 28. A method foramplitude modulation of an optical signal with a binary data signal asclaimed in claim 23, wherein the operating point control signal and theamplitude of the modulation signal are optimized by sweeping respectivefrequencies of the operating point control signal and the modulationsignal, and by control being carried out in each case based on a lock-inprinciple.
 29. A method for amplitude modulation of an optical signalwith a binary data signal as claimed in claim 21, the method furthercomprising the steps of: producing the recovered data signal bydemodulation of an optical received signal; deriving controlled signalsfrom the recovered data signal; supplying the control signals to acontrol device arranged at a transmission end; and transmitting controlsignals to a control device arranged at a receiving end, the controldevice controlling the modulator via a service channel.
 30. A method foramplitude modulation of an optical signal with a binary data signal, themethod comprising the steps of: supplying both the optical signal andthe binary data signal to a modulator for producing a transmissionsignal whose operating point is adjustable; demodulating thetransmission signal; converting the demodulated signal to a recovereddata signal; deriving controlled signals from the recovered data signal;supplying the control signals to a control device arranged at atransmission end; transmitting control signals to a control devicearranged at a receiving end, the control device controlling themodulator via a service channel; filtering out at least one harmonicfrequency of the recovered data signal; and using a signal at the atleast one harmonic frequency, which has been filtered out, to derive acontrol signal which controls an operating point of the modulator suchthat the at least one harmonic frequency reaches one of a minimumamplitude and a minimum power.
 31. A method for amplitude modulation ofan optical signal with a binary data signal as claimed in claim 30, themethod further comprising the steps of: selecting both a fundamentalfrequency and the at least one harmonic frequency from the recovereddata signal; and using control signals derived from the fundamentalfrequency and the at least one harmonic frequency for optimum adjustmentof the operating point.
 32. A method for amplitude modulation of anoptical signal with a binary data signal as claimed in claim 30, themethod further comprising the steps of: converting the binary datasignal to a modulation signal which is supplied to the modulator; andregulating an amplitude of the modulation signal at a value at which oneof the fundamental frequency and a control signal formed from thefundamental frequency and the inverted harmonic frequency reaches amaximum.
 33. A method for amplitude modulation of an optical signal witha binary data signal as claimed in claim 32, the method furthercomprising the step of varying one of the operating point and anamplitude of the modulation signal on a trial basis until thefundamental frequency has reached a maximum value.
 34. A method foramplitude modulation of an optical signal with a binary data signal asclaimed in claim 33, wherein the operating point and the modulationsignal are optimally set alternately.
 35. A method for amplitudemodulation of an optical signal with a binary data signal as claimed inclaim 32, the method further comprising the step of varying one of theoperating point and an amplitude of the modulation signal on a trialbasis until the fundamental frequency has reached a maximum value andthe harmonic frequency has reached a minimum value at the same time. 36.A method for amplitude modulation of an optical signal with a binarydata signal as claimed in claim 35, wherein the operating point and themodulation signal are optimally set alternately.
 37. A method foramplitude modulation of an optical signal with a binary data signal asclaimed in claim 32, wherein the operating point control signal and theamplitude of the modulation signal are optimized by sweeping respectivefrequencies of the operating point control signal and the modulationsignal, and by control being carried out in each case based on a lock-inprinciple.
 38. A method for amplitude modulation of an optical signalwith a binary data signal as claimed in claim 30, the method furthercomprising the steps of: producing the recovered data signal bydemodulation of an optical received signal; deriving control signalsfrom the recovered data signal; supplying the control signals to acontrol device arranged at a transmission end; and transmitting controlsignals via the control device and via a service channel to themodulator.
 39. A system for amplitude modulation of an optical signalwith a binary data signal, comprising: a modulator with an adjustableoperating point, the modulator having an output at which a modulatedtransmission signal is emitted; a demodulator, in a control loop,through which one of the transmission signal or a measurement signalwhich corresponds to the transmission signal is supplied in order toobtain a recovered binary data signal from it; a first filter, in thecontrol loop, which selects a fundamental frequency of the recoveredbinary data signal; at least one second filter, in the control loop,which selects at least one harmonic frequency of the recovered binarydata signal; at least one measurement transducer, in the control loop,for obtaining control signals from at least the first and secondfilters, wherein the measurement transducers are power measurementdevices; and a control device, in the control loop for controlling theoperating point of the modulator such that the amplitude of thefundamental frequency reaches for a maximum value; and the amplitude ofthe at least one harmonic frequency reaches for a minimum value.
 40. Asystem for amplitude modulation of an optical signal with a binary datasignal as claimed in claim 39, further comprising an adjustableamplifier for converting the binary data signal to a modulation signal,wherein the modulation signal is regulated at a maximum value at whichthe amplitude of the fundamental frequency is at a maximum.